Platform-specific main function abstraction and callback system.

This module provides a portable entry point system for applications that works across different platforms (Windows, Emscripten, iOS) by wrapping platform-specific initialization code.

Two usage modes are available:

• Standard mode: Define your own bj_main function.
• Callback mode: Implement lifecycle callbacks (begin, iterate, end).

Configuration Macros

• BJ_NO_AUTOMAIN: Define to provide your own main function.
• BJ_AUTOMAIN_CALLBACKS: Define to use the callback system instead of bj_main.
• BJ_AUTOMAIN: (Internal) Automatically defined on platforms requiring special setup.